fix: Examples update

[tmathern]

• Add an example that showcases v2 manifest signing + callback signer
• Keep old example (renamed to signer_info) showing signing using signer information and v1 manifest
• Use cawg assertions in more examples
• Add unit tests for V2 signing (because I want us to have the examples also be part of the unit tests)

[tmathern]

I believe this is the most minimal valid V2 manifest we can have.

[crandmck]

In this example, a callback signer does the signer:

[tmathern]

In this example, a callback signer is the signer is more accurate (is instead of does - because it is only one part that we use, we still provide the whole other plumbing to sign)